Jacinto L. Marchetti

Dynamic simulation of large boilers with natural recirculation

Abstract This paper presents a dynamic simulator of water-in-tube boilers with natural recirculation, the kind of equipment widely used in industries for steam generation either as a source of power or for providing heating capabilities in process plants. The development is based on a combination of two non-linear models, one for the evaporation in the vertical tubes and the other for the phase separation in the steam drum. An application is made to the boiler of a 30 MW thermoelectric power plant and the results are discussed. The dynamic responses of all variables show the consistency of the model representation with the expected behavior, including the effects of a PI level control adjusted using classic Ziegler–Nichols tuning rules.

Designing and tuning robust feedforward controllers

Abstract A model-based design and tuning procedure is proposed for feedforward controllers, which accounts for model uncertainties in SISO systems. Proper relationships for the analysis of feedforward–feedback control systems show that tuning the feedforward controller is not completely independent from the feedback-loop spectral characteristics. Similarly, fine tune of the associated feedback controller requires to be based on the residual disturbance remaining after the feedforward control action. Consequently, the simultaneous tuning is proposed for efficiently solving disturbance-rejection problems. Two application examples show that the robust combined tuning gives satisfactory results for different dynamics and different tuning requirements.

Modelling and parametric sensitivity of an annular photoreactor with complex kinetics

Abstract This paper reports the behaviour of a continuous annular reactor irradiated with a tubular ultraviolet source placed at its axis, and analyzes the case of a chemical reaction occurring in the homogeneous phase with complex kinetics. The reacting system is analyzed without resorting to the kinetic steady state hypothesis, thus allowing the computation of concentration profiles for the intermediate activated species. Three different models of radiation sources are considered: the line source model with emission in parallel planes, the line source model with spherica We have investigated and evaluated the influence of all significant parameters of the system on the reactant, intermediate species and product concentr

Model predictive control suitable for closed-loop re-identification

The main problem of a closed-loop re-identification procedure is that, in general, the dynamic control and identification objectives are conflicting. In fact, to perform a suitable identification a persistent excitation of the system is needed, while the control objective is to stabilize the system to a given equilibrium point. In this work, a generalization of the concept of stability, from punctual stability to (invariant) set stability, is studied to avoid the conflict between these objectives. Furthermore, a persistent excitation scheme is proposed to properly perform a closed-loop re-identification. Simulation results show the propose controller formulation properties.

Low-level flexible-structure control applied to heat exchanger networks

A low-level flexible-structure control is proposed for designing control systems capable of efficiently handling constraints on the manipulated variables of heat exchanger networks (HENs). Flexible-structure refers to the capability of the resulting control system to switch from one closed-loop structure to another in order to keep regulation, and low-level means that it can be configured in most distributed control systems. This control approach is useful to hold the operating point close to an optimum when optimal conditions are located on the constraints. The application example compares the approach with the use of a more involved strategy.

On Economic Optimality of Model Predictive Control

Model Predictive Control (MPC) is the most used advanced control strategy in the industries, mainly due to its capability to fulfill economic objectives, taking into account a dynamic simplified model of the plant, constraints, and stability requirements. In the last years, several economic formulations of MPC have been presented, which get over the standard setpoint-tracking formulation. The goal of this work is to provide, by means of application to a highly nonlinear plant, a comparison of different strategies, focusing mainly on economic optimality, computational burden, and economic performance.

Flexible-Structure Control - A Strategy to Handle Input Constraints

Abstract A new strategy is proposed for designing and implementing control systems capable to efficiently handle limiting values (constraints) of principal or preferred manipulated variables by combining the control action with a secondary or axiliary control variable. Flexible-structure control refers to the capability of the resulting control systems to switch from on closed-loop structure to another in order to keep regulation and/or to hold the operating point close to an optimum.

DETECTING STATIONARY GAIN CHANGES IN LARGE PROCESS SYSTEMS

Stationary process gains are critical model parameters for determining targets in commercial MPC technologies. Consequently, important savings can be reached by accessing an early prevention method capable of detecting whether the actual process moves away from the modeled dynamics, particularly by indicating when the process gains are no longer represented by those included in the model identified during commissioning stages. In this first approach, a subspace identification method is used under open-loop process condition to estimate the process gain matrix. The main reason for using the subspace identification (SID) method is that it works directly with raw data; it directly yields a multivariable state space model and has proved to be successful in dealing with multivariable processes and periodic batch-wise data collection. To detect significant changes in the estimator population, a monitoring sequence of hypothesis tests can be done through simple confidence limits directly on each gain estimator, or increasing the sensitivity by using the exponentially weighted moving average (EWMA) or the cumulative sum (CUSUM) algorithms. The objective of this aticle is to present a rational combination of inferential tools capable of detecting which gain of a multivariable model starts moving away from its original value. The anticipated knowledge of these events could provide a warning to process engineers and prevent targeting process conditions with wrong gain estimations. The regular follow-up of the gain matrix should also help to localize those dynamics needing an updating identification and reduce the frequency of time-consuming re-identification of the complete model.

COMBINING CONCEPTUAL AND REFERENTIAL REACTION METHODS FOR BATCH DISTILLATION CONTROL

Abstract The referential reaction empirically determines reduced-order models for designing and tuning feedback controllers dedicated to tracking non-stationary conditions in batch processes. Defining a feedback control system for a batch distillation implies first the selection of an appropriate tray temperature evolution. Then, the dynamics associated to the manipulated variable (distillate flow rate) is isolated from the main time-variable behavior desired for the operation, and used with available tuning rules for integrating systems. The effectiveness of this procedure is illustrated by implementing a feasible quasi-optimal recipe derived from pinch theory and valid for multicomponent mixtures.

Closed-loop implementation of optimal trajectories in batch distillation

Abstract In order to implement a quasi-optimal trajectory derived from pinch theory [1,2], a temperature tracking control system composed by an open-loop reflux ratio control plus a closed-loop correction for disturbance rejection is proposed. To adjust the closed-loop controller we use the referential dynamic reaction of the process [3] and tuning rules [4] that, though they were originally developed for dynamics valid in the neighborhood of stationary operating points, under this strategy they are useful in the neighborhood of a reference transient evolution like those occurring in batch distillation columns. The separation of the light component from its ternary mixture of alcohols is studied here.